Deep Inside a Giant – Centaurus A by Mike Sidonio

Before you dismiss this as just another photo of Centaurus A, you better look again. It’s way deeper… First discovered by James Dunlop on August 4, 1826 this incredible galaxy known as Centaurus A (NGC 5128) has been tickling the imagination of astronomers since John Herschel described it as “two semi-ovals of elliptically formed nebula appearing to be cut asunder and separated by a broad obscure band parallel to the larger axis of the nebula, in the midst of which a faint streak of light parallel to the sides of the cut appears.” in 1847. What makes this incredible galaxy tick? Step inside and let’s find out…

Regardless of the fact that J. Herschel pointed out NGC 5128’s unusual characteristics, it would be 102 years before astronomy really took this galaxy seriously – not because the science didn’t progress – but because there just wasn’t any large optical telescopes located in the southern hemisphere. However, things were about to change drastically in 1949 when the 80-foot radio antenna at Dover Heights, Australia went on-line. There astronomers John Bolton, G. Stanley, and Bruce Slee were the first to identify Centaurus A as a powerful radio galaxy – the first source to be linked to an extra-galactic hot spot.

But just how hot is it? Try a study done in July 2008 by Cuoco and Hannestad searching for ultrahigh energy neutrinos from Centaurus A and the Auger hot spot. “The Pierre Auger Collaboration has reported a correlation between ultrahigh energy cosmic rays (UHECR) and nearby active galactic nuclei (AGN) within Ëœ75Mpc. Two of these events fall within 3 degrees from Centaurus A (Cen A), the nearest AGN, clearly suggesting that this object is a strong UHECR emitter. Here we pursue this hypothesis and forecast the expected rate of ultrahigh energy neutrinos in detectors like IceCube. In our baseline model we find a rate of Ëœ0.4â€“0.6yr-1 events above a threshold of 100 TeV, the uncertainty of which is mainly related to the poor knowledge of the physical parameters of the source and details of the model. This situation will improve with detailed high energy gamma ray measurements of Cen A by the upcoming Gamma Ray Large Area Space Telescope (GLAST) satellite. This would make Cen A the first example where the potential of high energy multi messenger astronomy is finally realized.”

Now, let’s go back in time… Back to 1954 at Palomar Observatory’s twin telescopes with Walter Baade and Rudolph Minkowski. It was then the first proposal was made that the bar of dark dust bisecting the galaxy was the result of a merger between two galaxies – a giant elliptical and a small spiral. “The radio source Cygnus A is an extragalactic object, two galaxies in actual collision.” This simple observation was again confirmed in 2005 by Karataeva (et al); “We present the results of stellar photometry in eight fields of NGC 5128 (Cen A), a candidate polar-ring galaxy, obtained by reducing images from the Hubble Space Telescope archive. In all cases, the color-magnitude diagrams reached the red-giant region, and the distance to the galaxy was determined from the position of the tip of the red-giant branch (4.1 Mpc), in agreement with previous estimates. Comparison of the diagrams with theoretical isochrones indicates that the red supergiants in the dark lane region are metal-rich, which is atypical of the polar rings. Our results are consistent with the assumption made by several authors that the absorption of a less massive spiral galaxy by a more massive one is observed in NGC 5128.”

But, that’s not all that’s coming off Centaurus A. Massive amounts of X-rays have been detected as well, with the very first picked up in 1970 with the use of a sounding rocket and then confirmed by the UHURU satellite. The emission was very localized, but it wasn’t steady, it changed in intensities. Again, scientific curiosity was aroused and again, an answer was found – a black hole. According to the work of Marconi (et al): “We present new HST Space Telescope Imaging Spectrograph observations of the nearby radio galaxy NGC 5128 (Centaurus A). The bright emission line with longest wavelength accessible from HST was used to study the kinematics of the ionized gas in the nuclear region. The STIS data were analized in conjunction with the ground-based near-infrared Very Large Telescope ISAAC spectra to infer the presence of a supermassive black hole and measure its mass. We performed a detailed analysis of the effects on MBH of the intrinsic surface brightness distribution of the emission line, a crucial ingredient in the gas kinematical analysis. The observed velocity dispersion in our spectra can be matched with a circularly rotating disk and also the observed line profiles and the higher order moments in the Hermite expansion of the line profiles, h3 and h4, are consistent with emission from such a disk. To our knowledge, Centaurus A is the first external galaxy for which reliable BH mass measurements from gas and stellar dynamics are available and, as in the case of the Galactic Center, the MBH gas kinematical estimate is in good agreement with that from stellar dynamics. Thus Centaurus A ranks among the best cases for supermassive Black Holes in galactic nuclei.”

Yet, is that all there is? No. As early as 1972, gamma ray emissions from NGC 5128 were being explored. Which, according to the work of Ozernoy and Aharonian, may very well be tied to the black hole itself. “An analysis of the experimental data on nuclear gamma-ray lines from Cen A reveals essential energetic difficulties, associated with the usual interpretation of these lines as a result of interactions of subcosmic rays with interstellar gas; since the necessary instantaneous energy loss rate of the cosmic rays should reach tremendous values. These difficulties are eliminated if the gamma rays are produced in the relativistic non-isothermal plasma near a compact source of activity â€” such as a massive black hole or a magnetoid (spinar).”

But don’t stop there. By the late 1970, John Graham had also discovered an outer gas shell from the galactic merger – a shell which was studied again in 2008 by Stickel (et al): “Deep far-infrared (FIR) imaging data detected the thermal emission from cold dust in the northern shell region of NGC5128 (Centaurus A), where previously neutral hydrogen and molecular gas has been found. These observations are in agreement with recent theoretical considerations that in galaxy interactions leading to stellar shell structures the less dissipative clumpy component of the ISM from the captured galaxy can lead to gaseous shells. Alternatively, the outlying gas and dust could be a rotating ring structure resulting from an interaction or even late infall of tidal material of a merger in the distant past. With all three components (atomic gas, molecular gas, dust) of the ISM present in the northern shell region, local star formation may account for the chains of young blue stars surrounding the region to the east and north. The dust cloud may also be involved in the disruption of the large scale radio jet before entering the brighter region of the northern radio lobe.”

But, let’s get down here. The photo at the top of this page wasn’t taken with the Hubble. It didn’t go through Chandra. It was taken by a very dedicated amateur astronomer named Mike Sidonio who understood exactly what needed to be done to capture all the true beauty of this too-often photographed sky gem. Says Mike; “This unique and extremely deep colour image, compiled from nearly 20 hours of exposure with just a 6” telescope, was taken from a very dark sky in remote Australia. The image reveals the full outer halo of the peculiar radio galaxy Centaurus A (NGC 5128) in Centaurus including faint polar extensions extending from the top and bottom of the galaxy running diagonally. Also evident in this image is the extensive but extremely faint Milky Way nebulosity and dust known as “Galactic Cirrus” or “Integrated Flux” that permeates this entire region. Galactic Cirrus material lies just above the plane of our galaxy and is lit by the light of The Milky Way as a whole but due to its extreme faintness at 27mag/sq arc sec, is seldom seen in images, it is visible as faint patches of dusty looking nebulosity all over the image. The Cirrus nebulosity around Centaurus A is some of the faintest in the sky and is well below the natural sky brightness. Countless distant background galaxies of all shapes and sizes can be found scattered all over the field of view as well.”

But Mike isn’t just any astrophotographer. He’s won numerous Malin awards and Astro Awards. His work has been featured in magazines such as Sky & Telescope and Astronomy, as well as Astronomy Picture of the Day, and this single Centaurus A image is only a small fraction of the study Mr. Sidonio did on this subject. For those of you who are curious, I would highly suggest visiting Mike Sidonio’s Centaurus A Pages, where each individual image takes you on an ever deeper visual journey into this fascinating galaxy.

By Tammy Plotner
Tammy was a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.
(Tammy passed away in early 2015... she will be missed)

Thanks again Tammy for another fascinating article on one of the closest AGN galaxies to our own. I appreciate the detailed description of several aspects of this peculiar galaxy. With regard to its recent association with UHECRs, you may want to check out the recent 2008 paper by Zew et al (“Galaxies Correlating with Ultra High Energy Cosmic Rays” @ arXiv:0806.3470v1) which finds 20 out of 27 of the most energetic cosmic rays correlating with active galactic nuclei (including Cen A with one whopping 70 Eev event!). Definitely a monster lurks in the heart of this galaxy. Also, kudos to Mike Sidonio for the great, deep image. Thanks for the link to his Cen A pages. These images blew me away.

I was blown away when I started researching Centaurus A and realized there had to come a point where I had to stop or have a 25,000 word article. As Jon and Mike have mentioned, there’s even more to the story than what’s in the story!

Tammy…
Nothing wrong with having an indepth 3 or 4 part article on any given galaxy. In reality, I think it would be a good idea for those new to astronomy. Educate there is a lot more to these beauties than just bright lights, and the fact we are still learning more every month!